This invention relates to a frequency measuring apparatus for measuring high frequencies, for example, above several hundred MHz, and more particularly to a frequency measuring apparatus in which an input signal, a local oscillation signal and its harmonic signals are frequency mixed by a frequency converter and the input signal frequency is measured utilizing intermediate-frequency signals derived from the frequency converter.
The frequency of an input signal can be measured by directly counting its waves per unit time. For input signal frequencies higher than microwaves, however, there is no counter capable of directly counting such high frequencies, or if there is any, it would be extremely expensive. A method that has heretofore been employed to obtain the input signal frequency is to convert the input signal into an intermediate-frequency signal and then measure its frequency. Further, in such a case, in order to obtain the intermediate-frequency signal over a wide frequency range, use is made of a frequency converter or the so-called harmonic mixer capable of obtaining not only the difference between the input signal frequency and a local oscillation frequency, but also the differences between the former and higher harmonics of the latter. The input signal frequency is measured from the number of waves of the resulting intermediate-frequency signal.
In this case, it is necessary to know whether the input signal is frequency mixed with the local oscillation signal itself or its harmonic and, in the case of the latter, its order. This involves troublesome operations and rapid measurement cannot be achieved. In view of this, there has been proposed such a frequency measuring apparatus as set forth, for example, in U.S. Pat. No. 3,932,814 (assigned to the assignee of the present application and patented Jan. 13, 1976), in which the input signal frequency is calculated on the basis of the difference in time between the moments at which the same intermediate-frequency signal is obtained, with the local oscillation frequency varied. In particular, the order of the higher harmonics of the local oscillation signal, with which the input signal is frequency mixed to obtain the intermediate-frequency signal, is detected from the above-mentioned time difference. In this case, a high degree of linearity is required between a control signal for controlling the frequency of a local oscillator and its output frequency, and in addition, it is necessary to vary the local oscillation frequency over a wide band. However, such a local oscillator is very expensive.